1. Field of the Invention
This invention is directed toward measurement of density of material, and more particularly directed toward a system for measuring bulk density of material penetrated by a borehole, wherein the system comprises a source of neutron radiation and preferably two radiation detection spectrometers. The system can alternately be embodied to measure other material properties and to measure density of materials not penetrated by a borehole.
2. Background of the Art
Systems utilizing a source of radiation and a radiation detector have been used in the prior art for many years to measure density of material. One class of prior art density measuring systems is commonly referred to as xe2x80x9ctransmissionxe2x80x9d systems. A source of nuclear radiation is positioned on one side of material whose density is to be measured, and a detector which responds to the radiation is positioned on the opposite side. After appropriate system calibration, the intensity of measured radiation can be related to the bulk density of material intervening between the source and the detector. A second class of prior art density measuring systems is commonly referred to as xe2x80x9cback scatterxe2x80x9d systems. Both a source of nuclear radiation and a detector, which responds to the radiation, are positioned on a common side of material whose density is to be measured. Radiation impinges upon and interacts with the material, and a portion of the impinging radiation is scattered by the material and back into the detector. After appropriate system calibration, the intensity of detected scattered radiation can be related to the bulk density of the material.
Backscatter type systems have been used for decades to measure density of material, such as earth formation, penetrated by a borehole. The measuring instrument or xe2x80x9ctoolxe2x80x9d typically comprises a source of radiation and at least one radiation detector which is axially aligned with the source and typically mounted within a pressure tight container.
Systems which employ the backscatter configuration with a source of gamma radiation and one or more gamma ray detectors are commonly referred to as xe2x80x9cgamma-gammaxe2x80x9d systems. Sources of gamma radiation are typically isotopic such as cesium-137 (137Cs), which emits gamma radiation with energy of 0.66 million electron volts (MeV) with a half life of 30.17 years. Alternately, cobalt-60 (60Co) is used as a source of 1.11 and 1.33 MeV gamma radiation with a half life of 5.27 years. The one or more gamma ray detectors can comprise ionization type detectors, or alternately scintillation type detectors if greater detector efficiency and delineation of the energy of measured scattered gamma radiation is desired.
The basic operational principles of prior art gamma-gamma type backscatter density measurement systems are summarized in the following paragraph. For purposes of discussion, it will be assumed that the system is embodied to measure the bulk density of material penetrated by a borehole, which is commonly referred to as a density logging system. It should be understood, however, that other backscatter density sensitive systems are known in the prior art. These systems include tools which use other types of radiation sources such as neutron sources, and other types of radiation detectors such as detectors which respond to neutron radiation or a combination of gamma radiation and neutron radiation.
A backscatter gamma-gamma density logging tool is conveyed along a well borehole penetrating typically earth formation. Gamma radiation from the source impinges upon material surrounding the borehole. This gamma radiation collides with electrons within the earth formation material and loses energy by means of several types of reaction. The most pertinent reaction in density measurement is the Compton scatter reaction. After undergoing typically multiple Compton scatters, a portion of the emitted gamma radiation is scattered back into the tool and detected by the gamma radiation detector. The number of Compton scatter collisions is a function of the electron density of the scattering material. Stated another way, the tool responds to electron density of the scattering earth formation material. Bulk density rather than electron density is usually the parameter of interest. Bulk density and electron density are related as
xcfx81e=xcfx81b(2(xcexa3Zi)/M W)xe2x80x83xe2x80x83(1) 
where
xcfx81e=the electron density index;
xcfx81b=the bulk density;
(xcexa3Zi)=the sum of atomic numbers Zi of each element i in a molecule of the material; and
MW=the molecular weight of the molecule of the material.
For most materials within earth formations, the term (2(xcexa3Zi)/MW) is approximately equal to one. Therefore, electron density index xcfx81e to which the tool responds can be related to bulk density xcfx81b, which is typically the parameter of interest, through the relationship
xcfx81b=Axcfx81e+Bxe2x80x83xe2x80x83(2) 
where A and B are measured tool calibration constants. Equation (2) is a relation that accounts for the near linear (and small) change in average Z/A that occurs as material water fraction changes with material porosity, and hence changes with bulk density.
The radial sensitivity of the density measuring system is affected by several factors such as the energy of gamma radiation emitted by the source, the axial spacing between the source and one or more gamma ray detectors, and properties of the borehole and the formation. Formation in the immediate vicinity of the borehole is usually perturbed by the drilling process, and more specifically by drilling fluid xe2x80x9cinvadesxe2x80x9d the formation in the near borehole region. Furthermore, particulates from the drilling fluid tend to buildup on the borehole wall. This buildup is commonly referred to as xe2x80x9cmudcakexe2x80x9d. Mudcake, invaded formation and other factors perturbing the near borehole region can adversely affect a formation bulk density measurement. It is of prime importance to maximize the radial depth of investigation of the tool in order to minimize the adverse effects of near borehole conditions. Generally speaking, an increase in axial spacing between the source and the one or more detectors increases radial depth of investigation. Increasing source to detector spacing, however, requires an increase in source intensity in order to maintain acceptable statistical precision of the measurement. Prior art systems also use multiple axial spaced detectors, and combine the responses of the detectors to xe2x80x9ccancelxe2x80x9d effects of the near borehole region. This method is marginally successful since nuclear systems are inherently shallow depth of investigation. Depth of investigation can be increased significantly by increasing the energy of the gamma-ray source. This permits deeper radial transport of gamma radiation into the formation. Unfortunately, there are no isotopic sources emitting gamma radiation above 1.33 MeV which have a half-life sufficiently long for typical commercial use and which are reasonably inexpensive to produce. Accelerator sources have been used in the prior art to generate gamma radiation of energy greater than 10 MeV. These sources are, however, physically large, costly to fabricate, costly to maintain, and often not suited for harsh environments such as a well borehole.
This invention is directed toward a system for measuring density and other properties of material penetrated by a borehole. Alternately, the system can be embodied for material analysis in a variety of non-borehole environments. Configuration of the system is based upon the backscatter concept discussed in the previous section of this disclosure.
The sensor instrument or xe2x80x9ctoolxe2x80x9d comprises preferably an axially spaced source of radiation and preferably two axially spaced radiation detectors, which discriminate energy of radiation impinging upon the detectors.
The source is preferably a neutron source which emits, or induces within material being measured, gamma radiation with energy greater than energy obtainable with isotopic gamma ray sources. Further, the neutron-induced gamma source is dispersed within the material being measured, providing for a larger investigation depth than obtainable with prior art systems wherein a gamma ray source is located within a tool and not dispersed within material being measured. The dispersed source of gamma rays results from the reaction of transported neutrons with the materials surrounding the tool. The neutron source is preferably isotopic, although other types of radiation sources such as a neutron generator can be used. Conceptually, a source of gamma radiation can be used but an accelerator type gamma ray source would be required to obtain the desired high energy radiation. Furthermore, such a source has the disadvantage of being a point source outside of the material being measured rather than a dispersed gamma source within the material being measured.
A first suitable isotopic neutron source is americium 241-beryllium (AmBe), which produces alpha, gamma and neutron radiation. More specifically, the AmBe source produces gamma radiation at 4.43 MeV from the decay of carbon-12 (12C), which is produced by alpha radiation from americium interacting with beryllium. Alternately, plutonium-beryllium (PuBe) can be used with similar results. Details of the alpha-beryllium reaction will be discussed in a subsequent section of this disclosure. The 4.43 MeV carbon gamma radiation is much more energetic that previously discussed gamma radiation from 137Cs (0.66 MeV) and 60Co (1.11 and 1.33 MeV). A gamma-gamma back scatter density measuring system using 4.43 MeV radiation will obtain much greater penetration into material being measured that that obtained with the prior art 137Cs or 60Co sources. More importantly, however, AmBe induces a source of gamma radiation within the material being measured resulting in a deeper depth of investigation. Neutrons from the AmBe source enters the material being measured. The neutron radiation produces gamma radiation primarily by inelastic scatter and thermal capture reactions with nuclei within the material. This neutron induced gamma radiation, and to a lesser proportion the back scattered 4.43 MeV gamma radiation, is detected by the preferably two axially spaced scintillation type gamma radiation spectrometers. The spectrometer positioned closest to the source is hereafter referred to as the xe2x80x9cshort spacedxe2x80x9d detector, and the spectrometer positioned farthest from the source is hereafter referred to as the xe2x80x9clong spacedxe2x80x9d detector. Other types of detectors exhibiting spectral gamma ray response can be used. Alternate detectors include solid state detectors and gas filled detectors fabricated to be energy dependent.
A second suitable isotopic neutron source is Californium-252 (252Cf). 252Cf does not produce 4.43 MeV gamma radiation internally as does AmBe (and PuBe). Neutrons are, however, produced, enter the material being measured, and produce an (nxe2x88x92xcex3) radiation with energy spectral characteristics similar to the (nxe2x88x92xcex3) component generated by AmBe. Energies of a significant portion of the induced (nxe2x88x92xcex3) radiation are greater than energies obtainable with isotopic gamma ray sources. The induced gamma radiation is detected by the short spaced and the long spaced detectors. Fission gamma radiation is also produced and incorporated into the measurement.
The system embodied as a borehole device is conveyed along a borehole by means of a wireline or a drill string. Gamma ray spectra are measured in both long spaced and short spaced detectors. Spectra from each detector are split and integrated over preferably two energy regions or energy xe2x80x9cwindowsxe2x80x9d. The energy windows are preferably contiguous, with a low energy window extending from a few hundred keV to about 3.0 MeV, and the high energy window extending from about 3.0 MeV to about 7 to 10 MeV. Energy windows for the short spaced and long spaced detectors are preferably identical, although different window limits can be employed using appropriate normalization methods. It should be understood that the energy limits of both the low window and the high window can be varied as long as the low energy window is responsive to down scatter gamma radiation and the high energy window is relatively insensitive to down scattered gamma radiation. As an example, a low energy window extending from several hundred keV to about 2 MeV would encompass a large percentage of down-scattered gamma radiation and backscatter radiation.
A normalization factor is next computed, wherein counts recorded in the high energy window of the long spaced detector multiplied by the normalization factor equals the counts recorded in the high energy window of the short spaced detector. Counts recorded in the low energy window of the long spaced detector are multiplied by the normalization factor thereby yielding a normalized low energy counts for the long spaced detector. The count measured in the low energy window of the short spaced detector is then subtracted from the normalized low energy count from the long spaced detector thereby yielding a low window count difference. For a given bulk density of material being measured, the percent of down scatter radiation increases with detector spacing. Furthermore, the percent of down scatter radiation is proportional the bulk density of the material being measured. These factors combine so that the low window count difference varies as a function of material bulk density, with low energy count difference increasing with increasing material bulk density. This process is recited mathematically in equations (1) and (2) discussed previously.
The functional relationship between low window count difference and material bulk density is determined by (1) calibrating by measuring tool response in materials of known density, or (2) by suitable tool response calculations such as Monte Carlo calculations, or (3) by a combination of measured and calculated tool responses.